Water purification system
In the Kamioka mine, there is clean water flowing near the detector and it can be used freely in large quantities. This water is circulated through the water purification system. The purpose of this water purification system is:
- To keep the water transparency as high as possible.
Small dust, metal ions like Fe, Ni, Co, and bacteria in the water are removed. - To remove the radioactive material, mainly Rn, Ra, and Th. Especially, radon is a serious background for the solar neutrino analysis.
Figure1: The water purification system.
Fig 1 shows the water purification system. The water purification system consists of the following components,
 | 1  m
Filter |
| Heat exchanger: The water temperature rises due to the heat generated by pumps and PMTs. The temperature is kept at around 14  C to suppress bacteria growth. |
| Ion exchanger: To remove metal ions in the water. |
| Ultra-Violet sterilizer: To kill bacteria. The documentation states the number of bacteria can be reduced to less than  . |
| Vacuum degasifier: To remove gas resolved in the water. It is able to remove about 99% of the oxygen gas and 96% of the radon gas |
| Cartridge polisher: This is a high performance ion exchanger. |
| Ultra filter (UF) To remove small dust even of the order of nanometers. Fig 2 shows the resistance of the water sampled at the inlet and the outlet of the Ultra filter. This can be compared the chemical limit of  . |
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Figure 2: |
The resistance of the water sampled at inlet and outlet of the Ultra filter as a function of time. |
Fig 3 shows the number of particles after the UF. After the Ultra filter, the water is returned to the detector. The UF removes 10% of the water passed through. That water is recirculated through the water purification system again via the following equipment, which is shown by the dashed line in Fig 1.
| Buffer tank |
| Reverse osmosis |
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Figure 3: |
The number of various sizes of particles as a function of time. |
The water is taken from the top of the tank using a pump and returned to the bottom of the tank. Its flow rate is about 50 tons/hour. This system keeps the water transparency above 70m as described elsewhere.
